Air bags are widely used in passenger cars, light trucks and vans as auxiliary protection devices to seat belts for increased driver and occupant protection in collision situations. The air bag was designed to enhance the protection offered by seat belt systems, especially in higher speed front impact crashes where minor belt-induced injuries and serious head and chest injuries can still occur to occupants restrained only by seat belts.
In most collisions, the decision to deploy the air bag is not made until late in the crash event, after contact with an object when a shift of occupant position is likely. This leaves the occupant vulnerable to serious injury or death by the inflation or unfurling of the air bag as it deploys. This raises particular concerns when the occupants are children or the elderly who tend to be more physically susceptible to injury. One method of mitigating injuries caused by deployment of the air bag is to use a pre-crash sensor to detect an imminent collision and help reduce the decision time of air bag deployment.
Primarily for reasons of economy, crash detection technology is currently provided by the use of a single point, fully electronic sensor located in the passenger compartment. The function of the crash sensor is to measure the crash severity as it occurs by measuring changes in velocity and, if a threshold is exceeded, air bag inflation is initiated. The output of this sensor is processed in a variety of ways. It can be filtered to provide a measure of the basic crash pulse, analyzed to establish the rate of acceleration change, integrated to indicate velocity change, or analyzed for frequency content or other parametrics. These measurements are then evaluated by a fuzzy logic system to provide reliable determination of the severity of the crash in progress, thus allowing for deployment initiation as early as possible. However, this processing takes time. In low speed crashes which produce velocity changes at or near the deployment threshold, the decision to deploy the air bag is not made until very late in the crash event, typically 25-50 milliseconds after contact. In such lower speed crashes an unbelted occupant will move closer to the air bag before inflation when compared to an occupant in a higher severity crash. The close proximity to the deploying air bag increases the chance of severe injury or death to the occupant as the pellets explode to inflate the air bag, not only because of the force of the inflating air bag, but from the heat generated from the explosion as well. Thus, current crash sensing technology provides less than ideal protection to the occupant in slower speed crashes.
A wide range of sensing devices have been investigated for a pre-crash sensor, including ultrasonic, infrared, and microwave radar. Of these, microwave radar has proven to be the strongest candidate for pre-crash detection for the simple reason that microwave radar is less affected by most environmental conditions that exist in driving situations, unlike ultrasonic and infrared sensors.
Use of microwave radar for primary air bag deployment has been investigated in the past. However, these systems required the use of Fast Fourier Transforms (FFT) to extract the harmonic components, which typically cost hundreds of dollars, and complex processors to run the systems due to the complex calculations performed. Other systems that performed harmonic binning spent considerable time monitoring each harmonic amplitude to determine range. These systems also required complex processors to perform the calculations necessary to detect the range of an object. Further, given the number of calculations, these processors required considerable time to determine the range and necessarily lag behind the actual range of fast moving objects. Thus, the high cost of early systems as well as the inability of radar to determine target mass deemed the use of microwave radar unacceptable. These pre-crash sensors detected objects that were too distant to pose a threat, or objects that were not on a collision path with the automobile such as mailboxes, signs and sidewalks.
U.S. Pat. No. 5,826,216, which is commonly assigned with the present invention, and is incorporated herein in its entirety, discloses a system that effectively uses microwave technology to determine if and when a collision will occur. However, it does not determine the offset of the collision from the sensor, which can be useful in eliminating objects which are not on a collision path with the automobile and for enabling a passive restraint system of the automobile to react appropriately, depending on where on the automobile the collision will take place.